Typical municipal and industrial wastewater contains pollutants expressed as biochemical oxygen demand (BOD). In a conventional flow-through wastewater treatment system which employs an activated sludge process for pollutant removal, the reactor vessel containing the activated sludge is continuously aerated as the wastewater passes through the vessel. Continuous aeration serves two purposes, it delivers oxygen to the active biomass of the activated sludge to sustain its ability to remove biochemical pollutants and it creates a turbulent flow condition in the vessel to thoroughly mix the activated sludge with the wastewater.
Aeration is conventionally accomplished with a combination of blowers and diffusers as are known in the art. Thus, two functions are preformed by the same equipment, namely, mixing and aeration. One of the disadvantages of using the same equipment for both the aeration and the mixing functions is that energy, a significant cost component of wastewater treatment, is not efficiently used. The blowers and diffusers of the aeration system run continuously, even if continuous aeration is not required to sustain the activated sludge. In addition, aeration promotes the nitrification of ammonia nitrogen (NH3—N) into nitrate nitrogen (NO3—N) which, while beneficial during treatment, may need to be removed later in the treatment process through denitrification.
Another disadvantage of a dual use or function aeration and mixing system is that the wastewater treatment plant operator is unable to control the balance between various beneficial forms of nitrogen in the activated sludge reactor vessel. Continuous aeration prevents the periodic or phased operation of the reactor vessel in an anoxic condition. Anoxic conditions in the reactor vessel both reduce the energy demand of the treatment process and allow for denitrification to occur before the treated flow passes through the vessel. Thus, there is a need for a flow-through activated sludge treatment process that provides for the phased operation of the reactor vessel either with or without aeration.
In addition to aeration, the reactor vessel in a conventional flow-through treatment process continuously receives a flow of return activated sludge (RAS) which has been recycled from a downstream solids separation phase. The RAS is reaerated in the reactor vessel, again imposing energy costs on the treatment process. Moreover, by reaerating the RAS, a source of bacterial solids that provide a food source for organisms that preferentially expel phosphorus is wasted. Thus, there is a further need for a flow-through activated sludge treatment process that provides for the phased conditioning and redirection of RAS prior to its return to the reactor vessel.
Along with the reduction of BOD, wastewater treatment systems must also provide processes for biological nutrient reduction (BNR) which includes the reduction of nitrogen and phosphorus concentrations in the wastewater effluent. BNR processes may be designed as separate sludge systems that conventionally take place after secondary clarification of a first stage of treatment. Such downstream removal processes require the expense of separate equipment, energy and chemical conditioning. Thus, there is a further need for a flow-through activated sludge treatment process that provides the ability to selectively remove non-preferential biological nutrients in a single sludge treatment system.
In certain embodiments of the inventions described herein, mixing and aeration operations in the reactor vessel are performed independently by equipment such as the MixAir® products of Aqua-Aerobic Systems, Inc. Thus, it is a feature and an advantage of these inventions that wastewater treatment process operators have independent control over mixing and aeration within the reactor vessel. Independent control, based on observed or measured conditions in the reactor vessel, allows for the regulation of aeration power to conserve energy without sacrificing necessary mixing. Also, such control capability permits the scheduling of anoxic periods in the reactor vessel to allow for nitrate reduction (denitrification).
In other embodiments of the inventions described herein, a reactor vessel with independently controlled aeration and mixing can be combined with a RAS separation vessel such as a phase separator of the general type described and referenced in U.S. Pat. No. 5,942,108 which is incorporated by reference. Thus, it is a feature and an advantage of these inventions that the bacterial solids of the RAS can be concentrated and separated from the RAS supernatant. The thickened solids from the phase separator can be diverted to an anaerobic vessel to enhance the nutrient removal process while the supernatant (containing proportionately higher concentrations of oxidized nitrogen) can be returned to the reactor vessel.
In other embodiments of the inventions described herein, an anaerobic vessel is positioned upstream of the reactor vessel and is in fluid communication with the phase separator. The anaerobic vessel receives thickened solids from the phase separator as a uniquely concentrated food source for organisms that preferentially expel phosphorus. Also, the phase separator diverts the RAS supernatant away from the anaerobic vessel, thereby isolating oxidized nitrogen from the anaerobic cell. Thus, it is a feature and an advantage of these inventions that phosphorus removal from the influent waste stream is enhanced in the reactor vessel. The ability to enhance phosphorus removal through the introduction of concentrated RAS solids to the anaerobic vessel reduces the cost of BNR treatment equipment, treatment chemicals and energy.
Further features and advantages of the inventions will be apparent to those of ordinary skill in the field of the invention from the more detailed description of certain embodiments of the inventions as described below.